1. Field of the Invention
The invention relates to a method of manufacturing an endless pressure belt for use in a device for the continuous pressing and decatizing of woven and knitted fabrics and the like.
2. Description of the Prior Art
Pressure belts for the above purpose are, while subjected to longitudinal tension, wound around one or more cylinders or one effect roller as well as guide rollers, and exert directly or indirectly a pressure upon the material to be processed. In such pressure belts, in addition to pulling stresses, bending change stresses arise, which are produced by the constant changes between concave and convex bending in the area of the guide rollers.
These pressure belts, which have to be very strong and resistant, are used as pressure-creating members, which press the material to be treated with controllable surface pressure against an effect roller. In this connection, surface pressure values of up to 5.0 kg/cm.sup.2 are necessary to bring about the required pressing effect in the material. The pressure belt and effect roller must also be heated up to 120.degree. or 160.degree. C.
In one previously proposed decatizing device, a drive roller provides for the drive of the endless pressure belt, the surface pressure being obtained by a hydraulically-controlled tensioning roller. This decatizing device is better than rotary pressing machines since, because of the synchronous rotation of all rollers, no stretching on pressing occurs in the material. In addition, an about 30-fold pressing area is available, as a consequence of which the action time of the surface pressure, temperature and moisture is 30 times greater than in the pressing process on a rotary pressing machine.
When particularly intensive fixing is required, sufficient moisture must also be available for the pressing operation. This requirement is met by arranging a steaming area in the immediate vicinity in front of the pressing arrangement. The fixing and stabilising of the material then takes place in the area of the surface pressure zone on the circumference of the effect roller. In order that the moisture applied should remain constant also under the temperature change during pressing, the pressure belt must be impermeable. The evaporation process is then inhibited by the impermeable pressure belt, as a result of which a true sweating process develops in the fabric. As the temperatures of the effect roller release fixing effects in the fabric, a fixing sweating process is achieved as opposed to the known tensioning press treatment. Practical tests have shown that particularly in the upper pressure area and at temperatures of 130.degree. C. it is possible to achieve material effects which, at present, can be achieved only by boiler decatizing with the maximum application of pressure.
Therefore, the following demands are made upon the construction of a suitable pressure belt for effecting the described stretch-free fixing and pressing treatment:
(a) the pressure belt must be exceedingly strong and resistant; PA1 (b) the pressure belt must be impermeable; and PA1 (c) the pressure belt must be free from marks as surface pressure is applied.
That it is not altogether easy to achieve these aims cumulatively has been evidenced by development work over many years. Tests have also made it clear that, because of the requirement for a smooth, homogeneous surface free from markings, the size of the stretching plays a substantial part. Stretching values of 3% for tensioning loads of 150 kg/cm may not be exceeded, as, otherwise, unsatisfactory changes occur in the surface structure. Stretching values below 3%, however, are achievable only with special materials, such as steel and the like.
However, the use of smooth steel bands must remain out of consideration from the outset as, because of the bending change stresses, the guide rollers of the pressing and fixing device would require a minimum diameter of 1000mm. This is impractical as regards the space required and also the cost. Such belts must be welded so as to be endless which, again, leads to marks on the material at the places of connection.
It has been proposed to use endless pressure belts of steel wire rope fabric which, because of the required nonpermeability, are vulcanised at least on one side. However, such pressure belts have a relatively short working life. The main problem is the vulcanised covering coming loose. The cause of this is to be found in the permanent bending change stresses of the convex and concave bends. Also with this pressure belt construction, the diameters of the guide rollers have to be considerably greater, in order to reduce the loads and in addition also the bending stresses.
The search for workable solutions has returned to pressure belts with textile support members. These are thinner in cross-section and thus are also more elastic at the critical guide places. Because of non-permeability these endless supporting members must also be vulcanised. As a result of vulcanising there arises a first-class impermeable and smooth surface. But these ideal conditions are changed over to their opposite when tensioning loads are required, in which case an "elephant's skin" clearly forms on the mirror surface. These irregularities lead to markings on the material with, as a consequence, an unusable pressing result. In addition, the rubber tends to come off after a relatively short time of use.